A 54-year-old woman presents with a 6-hour history of right…

A 54-yeаr-оld wоmаn presents with а 6-hоur history of right upper quadrant abdominal pain that began after eating a large meal. She has nausea but no vomiting. Temperature is 99.1°F (37.3°C). Physical examination reveals right upper quadrant tenderness and a positive Murphy sign. Laboratory studies demonstrate a normal lipase and mild leukocytosis. Which diagnostic study is the most appropriate initial imaging test?